The conventionally known electronic musical instrument provides a waveform memory which pre-stores a musical tone waveform generated from a non-electronic musical instrument (hereinafter, simply referred to as "acoustic instrument") and the like. When a musical tone waveform is stored in the waveform memory, the tones of an acoustic musical instrument are converted to electric signals by means of a microphone, and the value of these electric signals is sampled at every moment, converted into digital data, and stored in memory. When musical tones are formed, the data in memory are read at a speed which corresponds to the pitch of the key which has been operated, these data are converted to an analog signal, and a tone is generated from a speaker.
When the entire musical tone waveform from the beginning of tone generation to the end thereof is stored in memory, the contents of the memory become considerably large. As the initial portion of the waveform changes in a complicated manner, the entire waveform is stored in memory, and as in the continuing and decreasing portions of the waveform a relatively monotonous waveform is repeated, a waveform of the duration of only one cycle (or a plurality of cycle) is stored in memory, and this waveform is repeatedly read and a musical tone is thus formed (looping processing). FIG. 21 shows one example of a musical tone waveform; in the drawing, Wao is the initial portion, and the other portion is the continuing portion. In the partial waveform memory method, the entirety of the initial portion Wao is stored in memory, and a single suitable cycle Wo is stored in memory as a repeating portion for the continuing portion.
A supplementary-pitch non-synchronous tone source is an example of an application of this partial waveform memory method. This tone source has advantages in that the waveform memory size can be kept small and there is no need to increase the address transfer speed of the waveform memory. Here, a tone source according to this method will be explained.
FIG. 22(a) is a waveform diagram of the tone source. First, the tone source is sampled by a sampling pulse of the fixed frequency (fs=fs1) shown in FIG. 22(b), and stored in memory. As a result, the waveform data shown in FIG. 22(c) are stored in memory. Next, the waveform data shown in FIG. 22(d) are interpolated (the diagram shows a linear straight-line interpolation) and resampled at a frequency fs2 (fs2 is not equal to fs1); the post-interpolation waveform data are then read. In other words, by resampling at a frequency of fs2 which is different from the frequency fs1, a change in the virtual sampling frequency is effected (see FIG. 22(f)). Next, when fs2 is made the same frequency as fs1, a tone having a new pitch is generated (see FIG. 22(g)).
In this way, it is possible to obtain a tone of a desired pitch from a limited sample; moreover, there is an advantage in that it is not necessary to create a sampling pulse which is synchronous with the pitch of the tone.